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Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
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https://doi.org/10.5194/acp-2020-986
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2020-986
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  13 Oct 2020

13 Oct 2020

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This preprint is currently under review for the journal ACP.

Continuous secondary ice production initiated by updrafts through the melting layer in mountainous regions

Annika Lauber1, Jan Henneberger1, Claudia Mignani2, Fabiola Ramelli1, Julie T. Pasquier1, Jörg Wieder1, Maxime Hervo3, and Ulrike Lohmann1 Annika Lauber et al.
  • 1ETH Zurich, Institute for Atmospheric and Climate Science, Zurich, Switzerland
  • 2Department of Environmental Sciences, University of Basel, Basel, Switzerland
  • 3Federal Office of Meteorology and Climatology MeteoSwiss, Payerne, Switzerland

Abstract. An accurate prediction of the ice crystal number concentration in clouds is important to determine the radiation budget, the lifetime, and the precipitation formation of clouds. Secondary ice production is thought to be responsible for the observed discrepancies between the ice crystal number concentration and the ice nucleating particle concentration in clouds. The Hallett-Mossop process is active between −3 °C and −8 °C and has been implemented into several models while all other secondary ice processes are poorly constrained and lack a well-founded quantification. During two hours of measurements taken on a mountain slope just above the melting layer at temperatures warmer than −3 °C, a continuously high concentration of small plates identified as secondary ice was observed. The presence of drizzle drops suggests droplet fragmentation upon freezing as the responsible secondary ice mechanism. The constant supply of drizzle drops can be explained by a recirculation theory, suggesting that melted snowflakes, which sedimented through the melting layer, were reintroduced into the cloud as drizzle drops by orographically forced updrafts. Here we introduce a parametrization of droplet fragmentation at high temperatures when primary ice nucleation is basically absent and the first ice is initiated by collision of drizzle drops with aged ice crystals sedimenting from higher altitudes. Based on previous measurements, we estimate that a droplet of 200 µm in diameter produces 18 secondary ice crystals when it fragments upon freezing. The application of the parametrization to our measurements shows high uncertainties, but the estimated number of splinters produced per fragmenting droplet (18–43) lies within the range of uncertainty if we assume that all droplets larger than 40 µm fragment when they freeze.

Annika Lauber et al.

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Annika Lauber et al.

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Short summary
An accurate prediction of the ice crystal number concentration (ICNC) is important to determine the radiation budget, lifetime, and precipitation formation of clouds. Even though secondary ice processes can increase the ICNC by several orders of magnitude, they are poorly constrained and lack a well-founded quantification. During measurements on a mountain slope, a high ICNC of small ice crystals was observed just below 0 °C, attributed to a secondary ice process and parameterized in this study.
An accurate prediction of the ice crystal number concentration (ICNC) is important to determine...
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